1. Field of the Invention
The present invention relates to an information storage medium and a process for producing the same. More particularly, the invention relates to an information storage medium having an air-sandwich structure which comprises two disc substrates joined together through spacers and a process for producing the same.
2. Description of Prior Art
Information storage media using high-energy-density beams such as a laser beam have been developed and put into practical use in recent years. The information storage medium (i.e., information recording medium) is called "optical disc" and can be used as video disc, audio disc and disc memory for large-capacity static image files and large-capacity computers. The optical disc basically comprises a recording layer provided on a disc-shaped transparent substrate made of a plastic material or a glass material. An undercoating layer or intermediate layer made of a high-molecular material may be provided on the surface (on which the recording layer is to be provided) of the substrate from the viewpoints of improving smoothness of the surface, adhesion between the substrate and the recording layer and the sensitivity of the resulting optical disc.
Various types of optical discs are known, and generally used in recent years is an optical disc having an air-sandwich structure comprising two disc substrates joined together through a ring-shaped inner spacer and a ring-shaped outer spacer and a recording layer provided on at least one substrate. Further, the joining of the substrates is made in such a manner that the recording layer is positioned inward, from the viewpoint of protection of the recording layer. As a process for producing such information storage medium of air-sandwich structure, a variety of processes have been proposed heretofore, and in those processes the spacers and the disc substrates are prepared from the same resin. However, the information storage medium produced by the conventional processes using the same resin for the preparation of the spacers and the substrates tends to be hardly reduced to satisfactory level in the axial acceleration in face deflection which greatly influences mechanical properties of the resulting optical disc, particularly follow-up properties of a pickup for the tracking of an optical disc on recording or reproducing procedure.
Japanese Patent Provisional Publication No. 62(1987)-173656 discloses that an information storage medium of an air-sandwich structure wherein a modulus of elasticity of its spacer is in the range of 20 to 90% of that of the disc substrate has good adhesion between the spacer and the substrate and high resistance to mechanical shocks and also shows reduced birefringence. In the information storage medium disclosed in the above publication, however, the axial acceleration in face deflection is not sufficiently reduced to a satisfactory level.